This invention relates to a process of producing magnesia from magnesium-containing carbonate ores, which comprises leaching with sulfuric acid, subsequently neutralizing the solution with magnesia, separating the undissolved impurities, crystallizing magnesium sulfate and thermally decomposing the latter to form magnesia and SO.sub.2, recovering sulfuric acid from the sulfur dioxide produced in the decomposing step and recycling the sulfuric acid to the leaching stage.
The magnesium chloride contained in sea water has mainly been used as a source of high-purity magnesia. An increase in concentration is succeeded by a thermal decomposing, which may be carried out, e.g., in accordance with German Patent 878,801 and results in a formation of magnesia and hydrogen chloride gas. This practice has the disadvantage that the resulting oxide has a purity of about 97% and is contaminated with boron compounds. A further disadvantage resides in the consumption of large quantities of energy required to increase the concentration of the originally highly dilute solution.
Another process of producing magnesia relies on the large deposits of carbonate ores. According to the U.S. Pat. No. 2,381,053, these ores are ground and in an aqueous medium are treated first with sulfur dioxide and then with air. This results in a formation of magnesium sulfite as an intermediate product and then of magnesium sulfate, which is filtered from solid residues and is concentrated by evaporation and finally thermally decomposed. The sulfur dioxide formed in the decomposing step is recycled to the first treating stage. This practice has the important disadvantage that the treatment with sulfur dioxide and the subsequent treatment with air result in solid-gas reactions which are relatively slow and require a high surplus of reactants, particularly in the second stage.
Another process has been proposed which also relies on magnesium-containing carbonate ores and in which the ores are leached in a surplus of hot sulfuric acid until the magnesium sulfate concentration exceeds 60% of the saturation concentration, free acid is neutralized with magnesia, the solid residues are separated, and magnesium sulfate is crystallized and is finally thermally decomposed (DOS 2,159,973). Sulfuric acid can be recovered from the gas produced by decomposing and may be re-used for leaching.
Whereas this process has considerable advantages compared to the one mentioned before, particularly because a much higher reaction rate is achieved during leaching, the further processing of the magnesium sulfate solution requires a multistep heat treatment, for instance by spray drying, final drying, and decomposing or by crystallization, drying, and decomposing. Besides, large quantities of dust which can be separated only with difficulty are formed by the thermal decomposing of magnesium sulfate from which all water has been removed. Moreover, only a rotary kiln has been described as a decomposing unit and has a low throughput rate per unit of the reactor volume and owing to the dissipation of radiant heat has a poor thermal efficiency.